Aromatic amines are important intermediates which have to be available inexpensively and in large volumes. Aniline, an aromatic amine of particular industrial significance, can be purified in an outstanding manner by the process according to the invention. Aniline is an important intermediate in the preparation of di- and polyisocyanates of the diphenylmethane series (MDI) and is prepared on the industrial scale generally by catalytic hydrogenation of nitrobenzene. For this purpose, it is necessary to build plants having very large capacities in order to be able to cover the enormous global demand. Preferably, the hydrogenation of nitrobenzene is conducted in the gas phase over fixed, heterogeneous supported catalysts, for example Pd on alumina or carbon supports, in fixed bed reactors at an absolute pressure of 2-50 bar and a temperature in the range of 250-500° C. under adiabatic conditions in cycle gas mode; see EP-A-0 696 573, EP-A-0 696 574 and EP-A-1 882 681. “Cycle gas mode” means that the uncondensable gases present in the crude reaction product (i.e. essentially hydrogen unconverted during the hydrogenation and any inert gases which have been added or formed through side reactions), possibly with the exception of small amounts branched off to keep the concentrations of further gaseous components constant in the cycle gas—for instance of ammonia formed as a result of deamination reactions on the catalyst—are recycled into the reaction.
In the preparation of aniline by hydrogenation of nitrobenzene, not only the target product but also water and organic secondary components are formed. In addition, according to the production process and state of operation, fractions of unconverted nitrobenzene may also be present. These organic secondary components and any unconverted nitrobenzene have to be removed down to residual contents of a few ppm before further use of the aniline. The organic secondary components and any unconverted nitrobenzene can be divided into two groups: a) the group of the “low boilers”, i.e. compounds or azeotropically boiling mixtures of individual components having boiling points below those of aniline (b.p.=184° C.), and b) the group of the “high boilers”, i.e. compounds or azeotropically boiling mixtures of individual components having boiling points above those of aniline. Nitrobenzene (b.p.=211° C.) accordingly forms part of the group of the high boilers. Because of the similarity of its boiling point to aniline, it is possible only with difficulty to remove phenol by a distillative route (see, for example, EP-A-1 670 747), the latter being an ever-present by-product in industrial hydrogenations of nitrobenzene.
A crude product stream of a gas phase hydrogenation of nitrobenzene thus generally consists of                (1) aniline,        (2) process water (which is the sum total of water formed in the reaction and any water present in the reactant gas stream),        (3) (uncondensable gases (uncondensable under customary industrial conditions for aniline workup) (excess hydrogen—optionally containing gaseous impurities, for example methane and any added inert gases, for example nitrogen added to improve selectivity (cf. EP-A-1 882 681), and any gaseous by-products, for example ammonia from deamination reactions),        (4) low boilers and        (5) high boilers (which may possibly also contain fractions of unconverted nitrobenzene).(1), (2), (4) and (5) are also referred to collectively hereinafter as “condensable constituents”.        
The state of the art is to free the aniline of all secondary components by distillation. Because of the high-boiling fractions in crude aniline (e.g. diphenylaminc having b.p.=302° C.), it is necessary for this purpose to vaporize the entirety of the aniline and condense it again at least once in the distillation, according to the reflux ratio, such that the distillation process incurs high energy costs.
A particular difficulty is the removal of those secondary components whose boiling points are very similar to that of aniline, because the distillation complexity here is considerable. In this connection, especially the removal of phenol (b.p.=182° C.) represents a great challenge for the distillation methodology, which is reflected in the use of long distillation columns with a large number of plates and high reflux ratios, with correspondingly high capital costs and energy expenditure. Compounds having phenolic hydroxyl groups, i.e. compounds bearing at least one hydroxyl group (—OH) directly on an aromatic ring, can generally be problematic in the workup of aniline. As well as phenol, which has already been mentioned, these include the various aminophenols. Although these are easier to remove by distillation because of the higher boiling point, they can cause both a viscosity rise in the column bottom and deposits in the distillation apparatus when bases, for example alkali metal hydroxide, are present in the distillation apparatus in order to optimize the phenol removal.
The purification of aniline is therefore not trivial and is of great industrial significance. Many approaches are dedicated particularly to the problems mentioned in connection with compounds having phenolic hydroxyl groups. The approach to a solution involves converting the compounds having phenolic hydroxyl groups, by reaction with suitable bases, to the corresponding salts which can be removed much more easily as nonvolatile compounds.
For instance, JP-A-49-035341, EP-A-1 845 079, EP-A-2 028 176 and EP-A-1 670 747 disclose processes in which an aromatic amine is distilled in the presence of a base. In this procedure, problems resulting from solids deposition, fouling and/or a significant viscosity rise in the course of distillation have to be prevented by complex and/or costly measures.
As an alternative to the removal of compounds having phenolic hydroxyl groups from aniline during the distillation, JP-A-08-295654 describes an extraction with dilute aqueous alkali metal hydroxide solution and subsequent distillation of the organic phase. Disadvantages of this process are the high NaOH consumption and the occurrence—as a result of the low concentration of the alkyl metal hydroxide solutions—of very large amounts of alkali metal phenoxide-containing wastewater, in addition to the high energy consumption in the distillation.
EP-A-1 845 080 describes a process for purifying aniline by extraction with aqueous alkali metal hydroxide solution of concentration >0.7% by mass, wherein concentration and temperature are adjusted such that the aqueous phase is always the lower phase in the subsequent phase separation. Optionally, to attain a desired product quality, the overall crude product can again be distilled before or after the extraction.
JP-A-2007217405 describes a process in which the phenol-containing aniline is contacted at least twice with aqueous alkali metal hydroxide solution in such a way that the concentration of alkali metal hydroxide in the aqueous phase is between 0.1% by mass and 0.7% by mass. This is followed by a separation of aqueous and organic phase and distillation of the organic phase.
The improvement of aniline workup is addressed in quite general terms by JP-A-2005 350388. A process is described in which a portion of the bottom product of the aniline distillation column is removed therefrom and converted separately to the gas phase, i.e. in a second evaporator other than the actual column evaporator. The gas phase thus obtained is recycled into the pure aniline column; unevaporable high boiler components are removed. A disadvantage of this process is that low boilers and water have to be removed upstream of the actual aniline distillation column, in a process which is complex in terms of apparatus, separately in a dewatering column by an additional distillation.
None of these publications mentioned so far addresses how it is possible to achieve reduction in the proportion of the aniline which has to be evaporated and condensed again in a distillation process. If the aniline to be purified originates from a gas phase process, it actually passes through two condensations according to the prior art: first of all, the reaction product obtained in gaseous form is condensed substantially completely, the aqueous phase is removed and the organic phase obtained is distilled, i.e. the desired product is (i) condensed, (ii) evaporated and (iii) condensed again, which is very energy- and apparatus-intensive and leads to thermal stresses on the aniline.
Only in the as yet unpublished application with reference number PCT/EP2011/068122 is this problem addressed. This describes fractional condensation of the aniline from a gas phase process, with introduction of the product stream originating from the partial condensation (PK) into the lower section of the distillation column between the lowermost stripping section and the subsequent section, and introduction of the product stream originating from the total condensation (TK) into the top of the distillation column above the uppermost rectifying section. Distilled aniline is withdrawn from the distillation column in a sidestream between the lowermost stripping section and uppermost rectifying section. This embodiment achieves the effect that the product stream originating from a total condensation need not be evaporated, but is instead freed of low boilers directly in the distillation column by stripping.
Since the removal of phenol by a distillative route is problematic, the process described requires several extractions, namely individual extractions for each product stream (design as per FIG. 3 in PCT/EP2011/068122). If, moreover, the ingress of salts into the distillation apparatus is to be prevented, each of these extractions additionally has to be followed downstream by a further extraction stage in which the product stream is washed with water. Alternatively, the extraction may also follow the distillation (design as per FIG. 4 in PCT/EP2011/068122), but already stripped product in this case has to be saturated again with water and may need to be subjected to another stripping. Another disadvantage is the strong coupling between the individual condensates, which arises through utilization of the product vapor ascending out of PK in the distillation apparatus in order to strip the product stream originating from TK. According to the product quality, this leads to increased energy intensity because, for example, in spite of a low content of low boilers in the product stream from PK, a high proportion of the overall product has to be run as PK into the lower section of the column and evaporated, in order to assure sufficient stripping of the product stream from TK. In other words, the separation of the overall crude product into TK and PK cannot always be effected as would be desirable for the purposes of an economically optimal workup; instead, it is always also subject to certain constraints which arise from the type of distillation. There are thus unsatisfactory coupling mechanisms in the case of division of the overall crude product in the fractional condensation into individual substreams and the subsequent workup thereof.
There is therefore a need for a process for purifying aniline originating from gas phase hydrogenations, in which only a minimum proportion of the aniline itself has to be evaporated and condensed again, and in which the removal of compounds having phenolic hydroxyl groups is achieved with maximum efficiency with minimum losses of valuable aniline. More particularly, unwanted coupling mechanisms in the division of the overall crude product in the fractional condensation into individual substreams and the subsequent workup thereof should also be reduced to a minimum.